We propose to study the morphology, physiology, and development of the malaria sporozoite, as well as the injection of the sporozoite into the mammalian host by the infected mosquito in the course of feeding. Electron microscopical, histological, histochemical, and autoradiographical procedures will be used. It is hoped that these investigations will increase our understanding of the cellular physiology of the sporogonic cycle of the malarial parasite, and of the invasion and early development of the sporozoite in the liver. Such knowledge may form a basis for a better understanding of the action of prophylactic antimalarial drugs, interferon inducing agents, and vaccination procedures against sporozoite-induced malaria. It may further help to increase our understanding of the epidemiology of mosquito transmission of malaria in nature.